Biosensing instruments are often used for the detection of various analyte levels in blood samples (e.g., glucose and cholesterol). Such instruments employ disposable sample strips with a well or reaction zone for receiving a blood sample. Analyte readings obtained from such instruments are dependent upon the ambient temperature that surrounds the sample well or reaction zone. Various prior art instruments employ external thermal sensors or make an attempt to control the temperature of the reaction zone. While external temperature sensors are capable of rapidly reacting to a temperature change, under certain circumstances that becomes a detriment rather than an attribute. For instance, if a biosensing instrument is small enough to be held in a user's hand, when that instrument is placed on a tabletop, a rapid temperature change can occur that will render invalid subsequent biochemical readings--until the ambient temperature reading has stabilized. If the biosensing instrument is battery-driven, it becomes impractical to control the temperature at the reaction zone as such action requires too great a power drain from the instruments battery.
The prior art includes a number of disclosures of biosensing instruments that employ temperature correction. In U.S. Pat. No. 5,108,564 to Szuminsky et al, a biosensing instrument is disclosed that measures glucose concentrations in blood. The instrument depends upon a reaction wherein glucose, in the presence of an enzyme, catalyzes a reaction of potassium ferricyanide to potassium ferricyanide. After the reaction has completed, a voltage is applied across a reaction zone and causes a reversal of the reaction, with an accompanying generation of a small, but measurable current. That current is termed the Cottrell current and, in dependence upon the concentration of glucose in the reaction zone, follows a predetermined curve during the reverse reaction. By determining the position of the curve, an indication of glucose concentration can be obtained.
European Patent application 047198682 of Tsutsumi et al discloses a blood glucose measurement system that employs disposable sample strips. The Tsutsumi et al system detects the presence of a blood sample by sensing a resistance across a pair of electrodes. It further employs plurality of sample-like strips, each having a specific resistance value which distinguishes it from other strips. Each of those strips has a particular application, i.e., for use during an adjustment mode of the instrument, during an error compensation mode; during a calibration mode; etc.
U.S. patent application Ser. No. 07/451,309, filed Dec. 15, 1989 to White and entitled "Biosensing Instrument and Method" and assigned to the same assignee as this application, teaches a biosensing instrument which employs the "Cottrell" curve relationship to determine glucose concentrations. In the White patent application, a ratio between current samples and times at which the current samples are taken is used to determine whether the current flow through a sample strip's reaction zone is, in fact, following the Cottrell relationship.
U.S. Pat. No. 4,420,564 to Tsuji et al. describes a blood sugar analyzer that employs a reaction cell having a fixed enzyme membrane sensor and a measuring electrode. The Tsuji et al. system includes several fail/safe procedures, one to determine that the reaction is taking place within specifically defined temperature limits and a second to determine if a reaction current remains within a predetermined range.
In the above noted prior art that indicates a need for temperature sensing, temperature values are obtained by temperature sensors and those sensed values are directly used. Variations in those sensed temperatures can create substantial variation in biochemical readings and cause erroneous outputs. Since such readings are of vital importance to the user and, if erroneous, may result in the mis-administration of medications, it is vital that erroneous readings be avoided. Thus, such biosensing instruments must include means for avoiding erroneous readings that result from erroneous ambient temperature inputs.
Accordingly, it is an object of this invention to provide a biosensing instrument with a method and means for providing accurate temperature values so as to enable proper analyte value indications.
It is another object of this invention to provide a biosensing instrument with a temperature sensor that is resistant to rapid temperature excursions as result of environmental changes, but still provides accurate ambient temperature values to enable analyte determinations.